Angioplasty balloon catheters are well known in the art. Basically they are comprised of a balloon portion and a catheter tube portion, the balloon portion being mounted on or attached to the catheter tube portion at or adjacent the distal end thereof. In use, the balloon portion is advanced through an artery, often over a guide wire which had previously been passed through the artery. The advancement is continued until the balloon is within a stenosis. It is then expanded by application of fluid pressure through the catheter tube.
Angioplasty catheters are normally of relatively small diameter and are preferably very flexible or "soft" to facilitate negotiating often very tortuous arterial paths. The balloon is advanced through the arterial tree by pushing on the catheter tube.
Sometimes, during the advancement of the balloon through the artery, it may encounter a lesion which impedes passage. In such a case, further force applied to the catheter, instead of producing further advancement of the balloon, merely causes the tube and/or the balloon, to crinkle, or collapse, or double up on itself within the artery. Most, often, the obstruction which leads to this is the very lesion causing the stenosis which the balloon is intended to expand.
Indwelling, on in situ guide wires have been used to try and overcome this difficulty. Because they are much thinner than the catheters and since they tend to be stiffer, they can often be guided through the small lumen of a stenotic region which a catheter alone might not be able to negotiate. With the guide wire having traversed the area of the lesion, a catheter passing over that wire can then find and negotiate that same path much more easily than if the guide wire were not there. Such guide wires also help the catheters passing over them to resist buckling as they pass through severely narrowed sections of the artery. Nevertheless, they have been only partially successful in ameliorating the tendency of the very flexible catheter tube to collapse, buckle or fold upon itself. That is because the axial force necessary to advance the balloon is still transmitted through the soft, flexible catheter tube. In addition, the need to pass over an indwelling guide wire imposes a minimum size limitation upon the diameter of the catheter tube and, as a result, on the collapsed profile of the balloon section. That minimum size can still be too large to enable the balloon to enter the lesion destined for treatment.
Moreover, the thrust recently has been toward making the in situ guide wire, not less flexible in order to enable it more easily to negotiate the tortuous path to the lesion. However, as the guide wire becomes more flexible, its ability to inhibit the collapsing or folding of the catheter tube diminishes.
Numerous attempts have been made over the years to design angioplasty catheters which are soft and flexible and yet can transmit axial forces without buckling. Among the most recent is reflected in U.S. Pat. No. 4,616,653 (Samson) which discloses a combination angioplasty catheter with a built-in guide wire. The system of the '653 patent employs a dual lumen catheter, with the inner lumen beginning at the distal end of the balloon, passing through the balloon chamber and then the full length of the catheter tube, ending at its proximal end. In addition, the inner lumen of Samson has at least three segments, a small diameter segment that passes through the balloon chamber, a large diameter segment that passes through the catheter tube and a transition segment between the other two.
While the device of the '653 patent may overcome some of the obstacles of the prior art devices, it is believed to have its own drawbacks. The use of coaxial tubes over the entire length of the device adds unnecessary rigidity and reduced flexibility in regions where it may not be needed or desired. Also, the use of a dual diameter inner tube, and the need to provide a transition zone with a taper that will not stretch when the guide wire taper is forced into it is believed to make the device of the '653 patent unnecessarily difficult and costly to fabricate.